Hemichordata

The hemichordata is a subphylum of the chordata (see chordate)
containing two types of animals: worm-like burrowing forms (called acorn
worms) and sedentary forms (known as pterobranchs). All are marine. The
burrowing forms have soft bodies divided into proboscis, collar, and trunk.
The proboscis and collar are distensible. The sedentary forms are colonial,
a number of individuals called zonoids being enclosed in a gelatinous "house."